1. Field of the Invention
The present invention relates to a wiring board including a light-emitting element mounted on the bottom surface of a cavity.
2. Description of the Related Art
In a wiring board having mounted thereon a light-emitting element, the emitted light can be collimated by adopting a structure in which a metal light reflection layer is arranged on the side surface of a cavity. A sealing resin is charged in the cavity so as to form a flat surface.
For example, a package for housing a light-emitting element has been proposed in which a ceramic frame having a through-hole is joined to a flat-plate-shaped ceramic base. A metal layer containing a refractory metal such as W or Mo and a metal plating layer containing Co are then formed in this order on the inner surface of the through-hole (see, e.g., JP-A-2004-228549, at pages 1-9 and FIGS. 1 and 4).
On the other hand, a photoelectric device has been proposed in which a photoelectric element is disposed in an opening space of a package. The side surface of the opening space is provided with a holding means to prevent peeling of a molding resin that is charged in the opening space (see, e.g., JP-A-11-74561, at pages 1-8 and FIGS. 1-5).
Incidentally, among the light reflection layers formed on the metal layer arranged on the side surface of the cavity in which the light-emitting element is mounted, a topmost metal plating layer of Ag or the like is thinner on a top end portion, for example, of the cavity side surface than on its middle portion.
Moreover, where the side surface of the cavity in which the light-emitting element is disposed is provided with a holding means that projects in cross section and serves to hold the molding resin (see, e.g., the photoelectric device of JP-A-11-74561 at pages 1-8, FIGS. 1-5), a plating liquid cannot easily circulate directly under the holding means in electroplating the metal layer formed on the side surface of the cavity with plating layers of Ni, Au, Ag, etc. As a result, those portions of the metal plating layers, in particular, the topmost Ag plating layer, which are formed on a top end portion of the metal layer directly under the holding means become thin. Also components of the underlying metal layer or metal plating layers may redeposit on the surface of the Ag plating layer. For example, in the case where an Ag layer is formed on a Ni layer, pinholes and the like tend to develop in a thin Ag layer. In that case, components of the lower Ni layer diffuse through the pinholes to redeposit on the surface of the Ag layer. The Ni forms an oxide (oxidation corrosion), and the Ni oxide gradually induces a change in color. In addition, by reacting with the Ni oxide, the Ag also induces a change in color. Such changes in color proliferate over time. That is, color changes are induced due to redeposition of an underlying plating layer in addition to variation in thickness of the respective plating layers, and such color change tends to originate and proliferate from the top end portion. Consequently, it becomes more difficult to efficiently reflect the light emitted from the light-emitting element.